The influence of the environment on gas surface density and star formation efficiency of cluster spiral galaxies is investigated. We extend previous work on radial profiles by a pixel-to pixel analysis looking for asymmetries due to environmental interactions. The star formation rate is derived from GALEX UV and Spitzer total infrared data based on the 8, 24, 70, and 160 μm data. As in field galaxies, the star formation rate for most Virgo galaxies is approximately proportional to the molecular gas mass. Except for NGC 4438, the cluster environment does not affect the star formation efficiency with respect to the molecular gas. Gas truncation is not associated with major changes in the total gas surface density distribution of the inner disk of Virgo spiral galaxies. In three galaxies (NGC 4430, NGC 4501, and NGC 4522), possible increases in the molecular fraction and the star formation efficiency with respect to the total gas, of factors of 1.5 to 2, are observed on the windward side of the galactic disk. A significant increase of the star formation efficiency with respect to the molecular gas content on the windward side of ram pressure-stripped galaxies is not observed. The ram-pressure stripped extraplanar gas of 3 highly inclined spiral galaxies (NGC 4330, NGC 4438, and NGC 4522) shows a depressed star formation efficiency with respect to the total gas, and one of them (NGC 4438) shows a depressed rate even with respect to the molecular gas. The interpretation is that stripped gas loses the gravitational confinement and associated pressure of the galactic disk, and the gas flow is diverging, so the gas density decreases and the star formation rate drops. We found two such regions of low star formation efficiency in the more face-on galaxies NGC 4501 and NGC 4654 which are both undergoing ram pressure stripping. These regions show low radio continuum emission or unusually steep radio spectral index. However, the stripped extraplanar gas in one highly inclined galaxy (NGC 4569) shows a normal star formation efficiency with respect to the total gas. We propose this galaxy is different because it is observed long after peak pressure, and its extraplanar gas is now in a converging flow as it resettles back into the disk.

VIVA Hi observations of the Virgo spiral galaxy NGC 4501 are presented. The Hi disk is sharply truncated to the southwest, well within the stellar disk. A region of low surface-density gas, which is more extended than the main Hi disk, is discovered northeast of the galaxy center. These data are compared to existing 6 cm polarized radio continuum emission, Hα, and optical broad band images. We observe a coincidence between the western Hi and polarized emission edges, on the one hand, and a faint Hα emission ridge, on the other. The polarized emission maxima are located within the gaps between the spiral arms and the faint Hα ridge. Based on the comparison of these observations with a sample of dynamical simulations with different values for maximum ram pressure and different inclination angles between the disk and the orbital plane, we conclude that ram pressure stripping can account for the main observed characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and is ∼200−300 Myr before peak ram pressure, i.e. its closest approach to M 87. The southwestern ridge of enhanced gas surface density and enhanced polarized radio-continuum emission is due to ram pressure compression. It is argued that the faint western Hα emission ridge is induced by nearly edge-on ram pressure stripping. NGC 4501 represents an especially clear example of early stage ram pressure stripping of a large cluster-spiral galaxy.

Ram pressure stripping of the multiphase ISM is studied in the perturbed Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction ∼100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA radio continuum observations at 6 and 20 cm are presented. We detect prominent extraplanar emission to the west of the galactic center, which extends twice as far as the other tracers of extraplanar material. The spectral index of the extraplanar emission does not steepen with increasing distance from the galaxy. This implies in situ re-acceleration of relativistic electrons. The comparison with multiwavelength observations shows that the magnetic field and the warm ionized interstellar medium traced by Hα emission are closely linked. The kinematics of the northern extraplanar Hα emission, which is ascribed to star formation, follow those of the extraplanar CO emission. In the western and southern extraplanar regions, the Hα measured velocities are greater than those of the CO lines. We suggest that the ionized gas of this region is excited by ram pressure. The spatial and velocity offsets are consistent with a scenario where the diffuse ionized gas is more efficiently pushed by ram pressure stripping than the neutral gas. We suggest that the recently found radio-deficient regions compared to 24 μm emission are due to this difference in stripping efficiency.

Deep scaled array VLA 20 and 6 cm observations including polarization of 19 Virgo spiral galaxies are presented extending previous work on the influence of the cluster environment on the radio continuum properties of Virgo cluster spiral galaxies. This sample contains six galaxies with a global minimum of 20 cm polarized emission at the receding side of the galactic disk and quadrupolar type large-scale magnetic fields. In the new sample no additional case of a ram-pressure stripped spiral galaxy with an asymmetric ridge of polarized radio continuum emission was found. In the absence of a close companion, a truncated Hi disk, together with a ridge of polarized radio continuum emission at the outer edge of the Hi disk, is a signpost of ram pressure stripping. Six out of the 19 observed galaxies (NGC 4294, NGC 4298, NGC 4457, NGC 4532, NGC 4568, NGC 4808) display asymmetric 6 cm polarized emission distributions. Three galaxies belong to tidally interacting pairs (NGC 4294, NGC 4298, NGC 4568), two galaxies host huge accreting Hi envelopes (NGC 4532, NGC 4808), and one galaxy (NGC 4457) had a recent minor merger. Tidal interactions and accreting gas envelopes can lead to compression and shear motions which enhance the polarized radio continuum emission. The resulting asymmetries are located within the Hi distribution. The 6 cm average degree of polarization only correlates with the galaxy mass or rotation velocity. In addition, galaxies with low average star formation rate per unit area have a low average degree of polarization. Shear or compression motions can enhance the degree of polarization. The average degree of polarization of tidally interacting galaxies is generally lower than expected for a given rotation velocity and star formation activity. This low average degree of polarization is at least partly due to the absence of polarized emission from the thin disk. Ram pressure stripping can decrease whereas tidal interactions most frequently decreases the average degree of polarization of Virgo cluster spiral galaxies. We found that moderate active ram pressure stripping has no influence on the spectral index, but enhances the global radio continuum emission with respect to the FIR emission, while an accreting gas envelope can but not necessarily enhances the radio continuum emission with respect to the FIR emission.

The influence of the environment on the polarized and total power radio continuum emission of cluster spiral galaxies is investigated. We present deep scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo spiral galaxies. These data are combined with existing optical, Hi, and Hα data. Ram pressure compression leads to sharp edges of the total power distribution at one side of the galactic disk. These edges coincide with Hi edges. In edge-on galaxies the extraplanar radio emission can extend further than the Hi emission. In the same galaxies asymmetric gradients in the degree of polarization give additional information on the ram pressure wind direction. The local total power emission is not sensitive to the effects of ram pressure. The radio continuum spectrum might flatten in the compressed region only for very strong ram pressure. This implies that neither the local star formation rate nor the turbulent small-scale magnetic field are significantly affected by ram pressure. Ram pressure compression occurs mainly on large scales ( > ∼ 1 kpc) and is primarily detectable in polarized radio continuum emission.